This invention relates to a method of through-casing gas detection; and to apparatuses for carrying out such a method.
In the technical field of oil production there are numerous important, technical reasons for identifying the presence of gas in a formation.
It is generally considered essential to acquire a good quality density log of a well, in order to provide for reliable gas detection.
Before completion of a well it is possible to obtain accurate density logs in open-hole. This is so even when there is mudcake in the well. Under that circumstance it is possible to compensate the density log for example using one or more of the techniques disclosed in “The Dual-Spaced Density Log—Characteristics, Calibration and Compensation”—Samworth, The Log Analyst, February 1992.
Until quite recently it was generally regarded as impossible to obtain the high-accuracy density logs following casing of the well.
However, it is possible to approximate the material of the casing to a mudcake of high density. Consequently it is possible to employ the spine/rib technique in the form specified in the above-mentioned paper by Samworth to obtain a log that is corrected for the effects of the casing.
A drawback of this technique, however, is that it cannot simultaneously correct for the irregular, cement-filled annulus encircling the steel casing following completion of the well. In addition, the dimensions of the annulus are such as to affect the accuracy of the spine/rib method.
When logging to identify gas, one usually compares the density-derived formation porosity with that derived from a neutron log. Following calibration of these measurements to read correctly in fluid-filled porosity, the presence of gas-filled porosity makes the neutron porosity log read low and the density porosity read high. When this happens the log traces deflect in opposite directions and when plotted appropriately cross over one another in a characteristic fashion. The crossover is therefore used to indicate the presence of gas.
Because the crossover depends principally on divergent log plot directions, it is possible to detect gas by comparing the character of the neutron and density logs without necessarily analysing in detail the absolute values of the logs.
However, when logging through steel casing, cavities located radially outwardly of the casing give rise to similar log plot crossovers as gas.
The mis-characterisation of cavities as gas-filled regions of the formation is a significant disadvantage of using neutron and density logs in combination to identify gas in a formation.
A further problem concerns the use of casing collars to join adjacent lengths of tubular casing together.
There are two kinds of casing joint in common use.
In flush jointed casing a threaded, tubular spigot of reduced diameter compared with the outside diameter of the casing protrudes from one end of a length of casing. The spigot is received in a threaded socket formed in one end of an adjacent length of casing. The socket has an inside diameter slightly greater than the inside diameter of the remainder of the casing.
When the casing lengths are screwed tightly together the effect is of a continuous wall of substantially constant thickness in the vicinity of the threaded parts.
Consequently a flush joint does not significantly affect the accuracy of gas detection logs.
A casing collar, on the other hand, that is an alternative means of joining lengths of a casing together, constitutes a steel annulus encircling the exterior of the casing in the vicinity of each joint.
The very nature of the casing collars emphasises the likelihood that they will influence the accuracy of a density log.
More specifically, casing collars cause the density log to read high density values over short distances.
Aside from securing adjacent casing lengths together, casing collars on the other hand are regularly spaced along the casing. Consequently their presence can be logged using a tool called a magnetic casing collar locator, that typically is deployed in conjunction with other downhole equipment to provide an accurate, absolute depth measurement.
There exist many oil wells whose productivity has declined in recent years. Such wells probably contain secondary hydrocarbon-bearing formations, that are not readily identifiable using conventional logging techniques. Hitherto the cost of comprehensively logging such wells using a sequence of techniques has been prohibitively high.
According to a first aspect of the invention there is provided a method of through-casing gas detection comprising the steps of:                (a) carrying out a plurality of respective neutron and un-compensated density logs, using neutron and density detectors, along a length of well;        (b) independently correcting each un-compensated density log for the dimensions and properties of the casing; and        (c) combining the thus-corrected density logs and the neutron logs so as to compensate for one or more regions outside the casing of artificially high density,        wherein the method includes the steps of:        (d) continuously dynamically calibrating the resulting compensated density logs against the neutron logs;        (e) carrying out a comparison between the thus calibrated logs and the neutron logs; and        (f) identifying gas crossovers from the said comparison.        
The method of the invention advantageously compensates for the effects of the casing, the cement annulus and any voids behind the casing. Thus the method offers an improved technique for the through-casing detection of gas formations.
The method can include the steps of:                (g) as necessary, detecting the presence of casing collars; and        (h) removing their effects from the density logs.        
More specifically the steps (g) and (h) can occur before step (c).
The foregoing features of the method of the invention additionally allow elimination of the effects of casing collars, when these are present. Conveniently the step (g) includes the step of:                (i) identifying as the effect of a collar an anomalously high value of the shorter spaced density log. This takes advantage of the effect of the casing collar in preferentially perturbing the shorter spaced density log data.        
The step (h) can also include the step of:                (j) for each collar effect detected in the logs, the substitution of non-collar affected data. Such non-collar affected data is that which is immediately preceding the collar effect in question.        
In other words, the first embodiment of the method of the invention substitutes data from regions of the well that do not coincide with the casing collars, for the anomalously high values that arise in the short spaced detector in such locations.
Step (b) includes one or more of the steps of:                (k) modelling the effect of the casing using a modelling database; or        (I) calibrating the logs using a casing calibration database.        
These techniques are advantageously reliable.
Alternatively, the step (b) may optionally include correcting the logs for effects of the casing using an iterative downhole calibration technique that is database-independent. While this method may be less precise, the absolute level of the log is not important in the method, as previously explained.
The foregoing technique may offer advantages in terms of computer processing power and response times.
Conveniently step (c) includes the step of:                (m) approximating the integrated geometric factor (G) of the well/density detector combination to an exponential function of the density log penetration depth.        
There is a detailed description of this technique in the paper by Samworth mentioned hereinabove. The entirety of this paper is incorporated herein by reference.
Preferably step (c) further includes (n) further approximating the exponential function to linear form.
There is a description of this technique in the aforementioned paper by Samworth.
Instead of the steps (m) and (n) specified herein, step (c) of the method of the invention may alternatively include the step (o) of:                approximating the integrated geometric factor (G) of the density measurement to a series of straight lines.        
The respective method steps (m) and (n) or (o) lend themselves to computation by different computational methods. It is possible for the logging engineer within the scope of the invention to use the method that is most appropriate to the prevailing circumstances.
In a particularly preferred embodiment of the invention, step (d) includes the steps of, after carrying out steps (g), (h), (i) and (j) as necessary,                (p) applying a long averaging filter to the neutron and density data resulting from the logs;        (q) calculating the difference between the thus filtered logs, by subtracting the density values from the neutron values; and        (r) adding the differences resulting from step (q) to the corresponding parts of the density logs.        
An advantage of this aspect of the invention is that the long averaged data is not skewed or otherwise perturbed by the high short spaced detector readings that would otherwise be present as a result of any collars surrounding the casing.
A further, optional feature of the invention involves the step of:                (s) screening spurious crossovers by comparing the crossovers to a further, lithology-sensitive log that is substantially insensitive to casing effects. Preferably the further log is a natural gamma ray log. Even more preferably, the natural gamma ray log is recorded simultaneously with the density and neutron logs.        
The invention as defined hereinabove allows highly accurate through-casing gas detection to take place. It is in addition desirable to carry out the method as defined hereinabove simultaneously with one or more of:                a neutron oil/water or water/gas contact log; and/or        an acoustic cement bond log.        
These method steps allow the economic and accurate logging of wells whose secondary hydrocarbon formations have hitherto been uneconomical to log.
In a preferred embodiment, the method is carried out using a single tool. Such a tool may contain all of the logging devices necessary to carry out the essential and preferred steps defined herein.
Even more preferably the tool is a compact battery/memory tool. Such a tool is self-powered and does not require a conventional armoured wireline for telemetry purposes.
By “compact” is meant a tool whose outside diameter is less than 2¼ inches. Such a tool is capable of more easily accessing narrow and otherwise difficult wells, than a tool of conventional diameter (ie. 3½ inches or greater).
The invention is also considered to reside in data acquired by the method steps defined herein.
According to a further aspect of the invention there is provided a well logging tool and data processing apparatus combination comprising a neutron sonde and a density sonde each secured in the tool, the density sonde including a caliper for urging the density sonde into contact with the interior surface of a casing string, the neutron sonde and the density sonde being operatively connectable to one or more programmable devices that are programmed to carry out at least steps (b)-(e) of claim 1 hereof.
Such a logging tool is of course advantageously suited to carrying out the method of the invention as defined herein.
Advantageous, optional features of the invention are defined in claims 21 to 28 hereof.